A common mask writer used in the art is known as a variable shape beam (VSB) mask writer. Such VSB mask writers require mask data with all angle edges approximated by staircases (stacked slits) in either a horizontal or vertical direction. That is, the original data representing a desired layout design is fractured into mask data in a VSB format that approximates the original data. The approximation uses 0, 45 or 90 degree edges, depending on the fracture setup and the angle of the edges in the original data. Skew error represents the differences between the fractured data in the VSB format output and the original data before fracturing.